Conventional laser printers such as those sold by Hewlett Packard and Canon, for specific examples, are designed to produce an image on paper or other print material, by placing overlapping dots at virtual positions defined by a digital raster. These virtual positions in the raster are known as "pixels", short for "picture elements". The degree of quantization of the raster is commonly referred to as the resolution; e.g., a printer having a 300 DPI (dots per inch) resolution, produces a raster of dots on 1/300 of an inch centers. Note that in this sense, "resolution" does not refer to how fine a line the printer can produce, and does not refer to the resolving of line pairs.
In laser printers, the image is first produced on a charged photoconductive material by sequentially scanning the photoconductor with a laser beam. As a line is scanned, the laser beam is energized during the extent of each pixel that corresponds to a desired dot in the image.
When the laser beam is energized, it discharges the portion of the photoconductor upon which the laser energy impinges. The sequential scanning by the laser produces a charge-pattern copy of the desired image on the photoconductor. The printed output is obtained by developing the charge pattern (attracting toner to it) and transferring the developed image to paper, or other print material.
The laser beam is only energized over the extent of a pixel; however, the resulting charge cluster that the beam produces on the photoconductor overlaps onto the pixel above, and the pixel below the pixel on the scan line. The charge cluster, which is roughly circular or oval in area, is distributed with non-linear, roughly bell-shaped intensity, which is higher at the center of the pixel. In the developing process that produces the output image, only the areas of the photoconductive material with charge above a certain threshold produce an image dot.
The image dots are made to overlap to preclude any of the background from showing through in solid multi-pixel areas, and to reduce the bead-like look of thin angled lines. To produce the overlap, the dot diameter is made to be significantly larger than one pixel; approximately two to three times as large. For example, a 300 DPI laser printer has dot diameters of approximately 1/100 to 1/150 (3/300 to 2/300) of an inch.
To obtain the virtual pixels that determine whether or not to energize the laser beam, the image to be printed is usually first rasterized; i.e., the source data, which is often analog, is quantized into non-overlapping virtual pixels. The pixel raster is also referred to at times as a bit map.
In the rasterizing process, graphic elements, such as continuous lines and character outlines are converted to pixel patterns that approximate the source shape. Continuous tone data, such as photographic data, is converted to fixed-size multi-pixel groupings that approximate the average gray value of the corresponding source data; thus, with binary pixels, a 6.times.6 multi-pixel grouping can simulate 36 levels of gray, and an 8.times.8 grouping can simulate 64 levels.
The laser printer resolution along a scan line can be easily doubled, since the laser can be turned on or off at a relatively arbitrary frequency, to produce the desired resolution. Resolution along the other axis is fixed; and determined by the interplay between the paper moving mechanism and the laser scanning speed.